In many medical situations, it is desirable and often necessary to implant medical devices that incorporate elongate structures. For example, elastomeric polymeric tubing, typically having a small diameter, is used in many medical applications and devices. Silicone rubber, especially cross linked silicone elastomer with silica filling, is the polymer of choice for fabricating tubing for use in many medical applications involving implantation. Other suitable elastomeric polymers include polyurethane, polyvinylchloride, polyesters and polyamides.
Implantable elongate elastomeric structures are typified by leads and catheters. Catheters prepared from elastomeric polymeric materials are used frequently in such routine procedures as the intravenous delivery of fluids, removal or drainage of urine or other fluids from compromised patients, chemical sensing using a variety of chemical transducers, monitoring cardiovascular dynamics, and treating cardiac and vascular disorders. Catheters provide the pathway to previously inaccessible body areas for both diagnostic and therapeutic procedures, thereby reducing the need for surgery. For example, double catheter systems are utilized for drug delivery or occlusion of blood flow to specific organs or tissues. In such procedures a rigid outer catheter and a buoyant, flexible inner catheter that can freely float in the blood stream are typically.
Examples of leads include cardiac pacing leads, tachycardia leads, and neurological leads. For example, a pacing lead utilizes a small diameter tubing such as less than 0.055 inch (1.40 mm) (OD) with an inner diameter (ID) of 0.35 inch (0.9 mm). In this type of lead, an elongate wire core (usually in the form of a coil) having a helical screw-in electrode at its distal end is placed inside small diameter tubing to provide a catheter-like device. The core wire is manipulated at the proximal end of this arrangement by the physician during implantation to screw the helical electrode into heart tissue and fix the lead in place.
As catheterization techniques have become more complicated, more demands placed on the performance of the catheter have increased. For instance, the paths that these catheters must take through the body are often long and tortuous, such as accessing the cranial vessels via the femoral artery. Silicone rubber tubing is especially useful for these applications because it is flexible, biocompatible, and allows for transfer of torque along its length. However, the polymeric materials from which catheters are made, such as silicone rubber, have a tacky surface upon exposure to an aqueous environment. This causes excessive friction, making placement of the catheter-like device in the body difficult. Further, these friction characteristics also make torque transfer through the tubing difficult thus, for example, making difficult the turning of the core wire which is preferably a torsion coil in the aforementioned "screw in" pacing lead to screw the helical electrode into tissue.
Plasma discharge has been used on polymeric tubing to modify the surface to improve its slip characteristics, but not by creating texture on the external surface of the tubing. For example, U.S. Pat. No. 5,593,550 (Stewart et al.) is directed to a plasma process for improving the slip characteristics of polymeric tubing on its outer diameter (OD) and inner diameter (ID) surfaces. U.S. Pat. No. 5,133,422 (Coury et al.) is directed to improving the slip characteristics of polymeric tubing on its OD surface by plasma treatment in the presence of a gas selected from the group consisting of hydrogen, nitrogen, ammonia, oxygen, carbon dioxide, C.sub.2 F.sub.6, C.sub.2 F.sub.4, C.sub.3 F.sub.6, C.sub.2 H.sub.4 C.sub.2 H.sub.2, CH.sub.4, and mixtures thereof. U.S. Pat. No. 4,692,347 (Yasuda) is directed to plasma deposition of coatings and to improving blood compatibility on both the OD and the ID surfaces of polymeric tubing by coating it under discharge conditions in a single chamber.
Plasma reactors are well-known in the art, examples of which are described by Yasuda, H., Plasma Polymerization, Academic Press (Orlando, Fla., 1985); and d'Agostino, R., Plasma Deposition, Treatment, and Etching of Polymers, Academic Press (San Diego, Calif., 1990). Typically, such plasma reactors use wave energy (RF or microwave) to excite plasma.
In general, a plasma reactor includes a glass reaction chamber that is fitted with a vacuum exhaust, gas inlets and at least one capacitively coupled electrode. In addition, the reactor is fitted with a pressure transducer and a mass flow controller for controlling and measuring the amount of gas being introduced into the reactor. The theory and practice of radio frequency (RF) gas discharge is explained in detail in 1) "Gas-Discharge Techniques For Biomaterial Modifications" by Gombatz and Hoffman, CRC Critical Reviews in Biocompatibility, Vol. 4, Issue 1 (1987) pp 1-42; 2) "Surface Modification and Evaluation of Some Commonly Used Catheter Materials I Surface Properties" by Triolo and Andrade, Journal of Biomedical Materials Research, Vol. 17, 129-147 (1983), and 3) "Surface Modification and Evaluation of Some Commonly Used Catheter Materials, II. Friction Characterized" also by Triolo and Andrade, Journal of Biomedical Materials Research, Vol. 17, 149-165 (1983).
Texturing of silicone surfaces has been achieved by transfer molding (photolithography) wherein a pattern is pressed into the silicone prior to curing. For example, flat stock silicone has been microtextured on one side by curing it on a microtextured glass mask or silicon wafer surface (J. Schmidt et al., Biomaterials 12, 385-389 (1991); patents describing this technology include (U.S. Pat. Nos. 5,219,361 and 5,011,494). However, transfer molding has not been used to create controlled texture on the external surfaces of elongate elastomeric structures; it is limited to planar-dimensional texturing. Microtexture on polyoxymethylene, PTFE and polyurethane surfaces has been achieved by natural ion bombardment etching (see von Recum et al., Tissue Engineering 2, 241-253 (1996)), however these surfaces are characterized by surface features having random size and distribution, rather than controlled texture comprising a deliberate array of surface features. Thermal evaporation has been used to form a random array of single-crystalline whiskers uniformly oriented with their long axes normal to the a polyimide substrate (J. Stahl et al., J. Vac. Sci. Technol. 14, 1761-1765 (1996)).
A number of patents have been reviewed in which plasma reactors are disclosed which use wave energy (RF or microwave) to excite plasma. Although not admitted as prior art, examples of plasma reactors and methods using the same can be found in the issued U.S. Patents listed in Table 1 below.
LIST OF U.S. PATENTS U.S. 5,593,550 01/14/1997 Stewart et al. U.S. 5,244,654 09/14/1993 Narayanan U.S. 5,223,308 06/29/1993 Doehler U.S. 5,133,986 07/28/1992 Blum et al. U.S. 5,133,422 07/28/1992 Coury et al. U.S. 4,948,628 08/14/1990 Montgomery et al. U.S. 4,927,676 05/22/1990 Williams et al. U.S. 4,846,101 07/11/1989 Montgomery et al. U.S. 4,752,426 06/21/1988 Cho U.S. 4,718,907 01/12/1988 Karwoski et al. U.S. 4,692,347 09/08/1987 Yasuda U.S. 4,488,954 12/18/1984 Choe et al. U.S. 4,261,806 04/14/1981 Asai et al.
Current texturing methods are, however, impractical for achieving texture, particularly microtexture characterized by controlled spacing of surface features, on nonplanar surfaces such as long continuous lengths of tubing surfaces, especially silicone surfaces. It is not possible to use transfer molding to form small patterns on the surface of nonplanar materials, such as long lengths of tubing. Ion beam etching does not allow for controlled or patterned spacing of surface features and, moreover, has not been demonstrated on silicone surfaces. There is, therefore, a need for a process of forming controlled microtexture on an elongate elastomeric surface.